In prior art patent documents U.S. Pat. No. 4,209,689 to G. Linford for “Laser Secure Communications System”, WO 2007/036937 for “Directional Light Transmitter and Receiver”, WO 2009/083990 for “Wireless Laser Power Transmitter” and WO 2012/172541 for “Spatially Distributed Laser Resonator”, the latter three having a common inventor with the present application, there are described various distributed resonator laser systems. All of the distributed resonator systems described in that prior art include a transmitter which incorporates a gain medium and a retro reflector acting as one mirror of the lasing system, and a receiver which also incorporates a retro reflector which can act as the second mirror of the lasing system and an output coupler for extracting the energy from the resonator.
In the case of WO 2012/172541, various optical elements are also incorporated to manipulate, monitor, and control the beam. Additionally, there is also taught therein elements such as beam blockers and irises, for blocking or avoiding unwanted reflections from the receivers. Such reflections could be a potential safety hazard.
The receivers described in the prior art may not be optimum for use in modern portable electronic devices, such as cellular phones and tablets, or even laptop PCs, as they are generally too large, and especially too deep. Furthermore, they may be unsuitable for use in the consumer environment, since, given the power requirements of cellular phones and other portable electronics devices, which is reflected in the power level circulating in the distributed laser cavity, they may not fulfil safety regulations. This is particularly so in an environment where finger prints, dirt, spilled liquids and the like are a reality of life, and their presence on the surface of the light receiving element of the receiver may compromise any safety measures taken to prevent stray laser emission.
Such systems can potentially be used to transfer energy to a remote device, such as a cellular telephone, and charge it. Eliminating or reducing the need to charge the phone by a cord, or by being placed on an inductive charging mat, therefore extends the time a device can operate between connections to a wall charger or being placed on a static charging mat.
As an example of the requirements of such a receiver, a receiver incorporated into the body of a cellular telephone, aimed at charging it, would typically need to have the following limiting parameters:    (a) Thickness preferably less than 6 mm.    (b) Radiation emissions limited, such that they are not a safety hazard. Typically, they should be less than the levels specified in the IEC60825 standard.    (c) Capability of generating at least 1 W of power    (d) A field of view of at least ±30 degrees, where the field of view is the sum of all angular directions from which the receiver could receive power, if a properly aligned transmitter were to transmit power thereto.    (e) The transmission range should be at least 3 meters.
Some of the above characteristics are more and some less critical. The first two criteria are perhaps the most important. The thickness is mandated by the thickness of mobile electronic devices such as cellular telephones. Unless the thickness is limited to that of these devices, typically 6 mm, the receiver solution may not be commercially acceptable. Therefore it is important to provide receivers sufficiently thin for inclusion within the thickness of such portable devices, where the field of view of the receiver faces in a direction generally towards the normal to the large surface of the device, since that is the position in which such devices are usually held. With regard to criterion (b), the front surface of the optical input element of such devices, on which light impinges before entering the receiver, is a particularly problematic feature, since reflections from that front surface may be a main source of safety problems with the receiver. Therefore, it would be important to provide an optimized coating for the front surface of such a receiver input element, to prevent or reduce such reflections to meet the relevant safety standard maximal permissible exposure (MPE). Furthermore, the optical beam blocking arrangement used should be such as to prevent unwanted reflections above the MPE from being directed in directions other than back to the transmitter unit.
Such receivers typically consist of a lens and a partially reflective mirror at its focal plane, such as can be seen in FIG. 7 of the above referenced publication WO 2012/172541. The front surface of the lens reflects some of the light impinging upon it, and such reflection can be controlled and minimized but cannot be completely eliminated. Such systems are limited in the amount of power they could safely transmit, as some of the power will always be reflected by the front surface. In such prior art systems, this surface is coated with an anti-reflection (AR) coating for at least some of the following reasons:    1. Safety—Optimizing the system for higher power transmission requires very low reflection from the front surface so that it would not exceed the maximal permissible exposure (MPE), such as that mandated by IEC60825. This would require having the front surface coated with anti-reflective (AR) coating, reflecting as little as possible light of the incoming light and therefore creating the least hazard.    2. Increasing efficiency and usefulness—Such AR coating would also improve the overall system efficiency. Reflections from the front surface represent a power loss, and losses reduce the system's efficiency dramatically as they compete with output coupling. To achieve minimal losses, the front surface should have a coating having a reflection as low as possible.    3. Increasing field of view—since symmetrical receiver configurations create the widest field of view, as they are indifferent to the direction the beam comes from, the front surface of the input lens of the receiver should be a spherical or nearly spherical surface. The radius of curvature of such a surface should be chosen to be minimal as this would serve to make the reflected beam intensity decrease rapidly with distance from the receiver as the beam diverges, so that it poses reduced risk after a short distance.
For a receiver with a large field of view, there are additional specific advantages for the use of such an AR coating:    (a) Without an AR coating, such a surface would have different Fresnel reflection/transmission properties at different angles and this may distort the beam's wavefront.    (b) The large field of view makes it difficult to use other means such as beam blockers to block the light reflected from the front surface. Such physical beam blockers are simpler to design when the field of view is small.
However, the use of an AR coating on a domestically used device such as a cellphone is problematic, since in such a consumer environment, dirt, spilled liquids, dust, fingerprints, and similar layers on top of the AR coating will amend the effectiveness of the AR coating, leading to higher reflections, and secondly, may eventually lead to degradation and peeling of the AR coating, again compromising safety. Therefore, alternative solutions must be found in order to reduce potentially dangerous reflections from the input optical surface of the receiver.
Other alternative solutions have been proposed, but each of these solutions has its own disadvantages:    1. Increasing the beam diameter so much that reflection from the receiver lens would be at a size and intensity so that it would be safe. Typically, a beam diameter of more than 7 mm. would be required, which is the standard aperture used in IEC60825. However, using this technique, the size of the beam needed to transmit the power required by a mobile device may be bigger than the entire area available for a receiver on the mobile device! For example, to supply power in the range of between 1 and 5 W, as needed to charge cellphones of various sizes, and using 20% photovoltaic efficiency, an optical power in the range of at least 5 to 25 W is needed. Taking into account that an uncoated glass surface reflects about 4% of the power incident on it, the range of optical power that needs to be transmitted ranges from at least 5.2 to 26 W. The MPE for 1400 nm light according to the IEC60825 (2nd edition) is of the order of 40 mW/cm2, Taking into account the field of view required (at least ±30 degrees) a beam having a diameter of approximately 3 cm would be required to charge the phone. Such a large beam cannot be input through the surface available for that purpose in a modern cellphone.    2. Using a beam block, typically made of an absorbing “black” material which essentially absorbs 100% of the light received by it. Its surface is typically a non-flat diffuser, so that light reflected by it would be diffused and pose minimal risk. However, using a beam block, of the type that is described in the above referenced WO 2007/036937, involves a significant thickness increase of the receiver as can be seen in FIG. 1 of that application. A beam block would have to be block any reflected beams within the field of view (FOV), which are typically reflected at a wider angular spread compared to the FOV, but at the same time, not block any beam within the field of view. Such a beam block height would have a very significant impact on the overall receiver height.    3. Diffusing the beam impinging on the receiver lens front surface—however this solution is not suitable for use inside a laser resonator, as it would distort the laser beam in a way that would not allow the wave front to be recreated after a round trip.
In the light of all of the above described disadvantages, it is clear that alternative solutions must be found in order to reduce potentially dangerous reflections from the input lens of the receiver.
Furthermore, in such prior art receivers, a single photovoltaic cell is generally placed directly behind the back mirror output coupler of the system, at a suitable distance so that the beam is spread to the diameter where maximal efficiency of the photovoltaic cell is achieved. This also increases the overall depth of the receiver, which is disadvantageous for such an application. It is clear that the prior art positioning and configuration of the photovoltaic cell(s) has a number of disadvantages, the trade-off between which is always a compromise to performance.
There therefore exists a need for a thin receiver unit of a distributed laser resonator which overcomes at least some of the disadvantages of prior art systems and methods.
The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.